1. Field of the Invention
The present invention relates to a loss point detecting method and a distributed Raman amplifier applying the same, and, in particular, to a loss point detecting method used in an optical system and a distributed Raman amplifier applying the same.
2. Description of the Related Art
Recently, a demand for communications rapidly increases in many countries along with a sharp spreading of the Internet or a development of multimedia society. For the purpose of catching up therewith, introduction of a backbone optical transmission system applying a WDM (wavelength division multiplexer) technology has been proceeded with, whereby increase in the transmission capacity is directed to.
In order to increase the transmission capacity, various methods such as time-division multiplexing, wavelength division multiplexing and so forth are considered, and, in particular, the method of wavelength division multiplexing is expected as being applied for the purpose of further increasing the transmission capacity with a seek for wider-band wavelength and a study concerning wavelength separation. However, in case of applying the technology for a use of long-distance transmission, an optical signal is attenuated there, and, thus, repeating or amplifying thereof is needed on the way.
There are two types of methods for amplifying an optical signal in an optical transmission path, i.e., of a stimulated emission type and of a Raman scattering type, and a distributed Raman amplifier (DRA) which applies the Raman scattering type amplification has been taken into a practical use widely in that it is possible to freely select the amplifying wavelength.
A WDM optical transmission system has a transmission station which generates a wavelength-multiplexed signal light, an optical transmission path transmitting the signal light generated by the transmission station, and a reception station which receives the signal light thus transmitted. Further, as needed, repeaters amplifying the signal light may be provided in the optical transmission path.
FIG. 1 illustrates a distributed Raman amplifier in the related art used in such a WDM optical transmission system (see Japanese laid-open patent application No. 2000-314902, for example). In the configuration shown, a signal light having the wavelength of 1.55 μm, for example, is transmitted through an optical fiber 10. This signal light is applied to a band separation optical coupler 12. Also, to the band separation optical coupler 12, an excitation light with the wavelength of 1.45 μm is supplied from an excitation light source 14 via an optical coupler 16. The signal light amplified optically with the supply of the excitation light is then sent out to a subsequent optical fiber via an optical coupler 18.
In the optical coupler 16, the excitation light from the excitation light source 14 is separated, and the power of the excitation light is monitored by an excitation light monitor 20, and the thus-obtained monitoring signal of the power of the excitation light is supplied to a control circuit 22. The signal light separated by the optical coupler 18 is monitored by a signal light monitor 24, and the thus-obtained monitoring signal of the power of the signal light is supplied to the control circuit 22. The control circuit 22 adjusts the excitation light power which the excitation light source 14 originally outputs according to the thus-obtained respective monitoring signals of the excitation light power and signal light power.
The distributed Raman amplifier (DRA) can perform amplification in an arbitrary wavelength zone, by appropriately setting the wavelength of the excitation light, and also, is advantageous in that the optical amplifying medium can be used also as the optical transmission path. Assuming that ‘go’ denotes the Raman gain factor, Pi denotes the applied excitation light power, Aeff denotes the nonlinear effective cross-sectional area, and Le denotes the DRA effective length, generally, the DRA gain Gr is expressed by the following formula (1), and the DRA effective length Le is expressed by the following formula (2);Gr=exp[(goPiLe)/(2Aeff)]  (1)
                              L          ⁢                                          ⁢          e                =                              ∫            0            L                    ⁢                                                    P                ⁡                                  (                  z                  )                                            /              P                        ⁢                                                  ⁢            i            ⁢                                                  ⁢                          ⅆ              z                                                          (        2        )            
There,P(z)=Pi·exp[−α(L−z)]
There, α denotes a constant, and ‘z’ denotes a relevant position along the optical fiber 10 assuming that L denotes the position of the band separation optical coupler 12.